A switch mode power converter (also referred to as a “power converter”) is a power supply or power processing circuit that converts an input voltage waveform into a specified output voltage waveform, which is typically a well-regulated voltage in electronic device applications. Power converters are frequently employed to power loads having tight voltage regulation characteristics such as a microprocessor with, for instance, a bias voltage of one volt or less provided by the power converter. To provide the voltage conversion and regulation functions, power converters include a reactive circuit element such as an inductor that is periodically switched to the input voltage waveform at a switching frequency that may be on the order of ten megahertz or more by an active switch such as a metal-oxide semiconductor field-effect transistor (“MOSFET”) that is coupled to the input voltage waveform.
A power converter configured to power an integrated circuit such as a microprocessor formed with submicron size features is generally referred to as a “point-of-load device,” and the integrated circuit is typically located close to the point-of-load power converter to limit voltage drop and losses in the conductors that couple the devices together. In such applications, a point-of-load power converter may be required to provide substantial current such as ten amperes or more to the integrated circuit. As current levels for integrated circuit loads continue to increase and the bias voltages decrease with on-going reductions in integrated-circuit feature sizes, the size of the power converter and its power conversion efficiency become important design considerations for product acceptance in challenging applications for emerging markets.
A recent development direction for reducing the size of point-of-load power converters has been to integrate the magnetic circuit elements therein, such as an isolation transformer or an output filter inductor, onto the same silicon substrate that is used to form the integrated control and switching functions of the power converter. These design directions have led to the development of micromagnetic devices with conductive and magnetic structures such as conductive windings and magnetic cores with micron-scaled dimensions to complement the similarly sized elements in logic and control circuits and in the power switches. The integrated magnetic circuit elements are therein produced with manufacturing processes and materials that are fully compatible with the processes and materials used to produce the corresponding semiconductor-based circuit components. The result of the device integration efforts has been to produce single-chip power converters including planar inductors and transformers capable of operation at the high switching frequencies that are necessary for point-of-load power converters to provide the necessary small physical dimensions.
As an example of a process to form a magnetic device that can be integrated onto a semiconductor substrate, Feygenson, et al. (“Feygenson”), in U.S. Pat. No. 6,440,750, entitled “Method of Making Integrated Circuit Having a Micromagnetic Device,” issued Aug. 27, 2002, which is incorporated herein by reference, describe a micromagnetic core formed on a semiconductor substrate by depositing Permalloy (typically 80% nickel and 20% iron) in the presence of a magnetic field. Dimensions of the core are designed using conformal mapping techniques. The magnetic field selectively orients the resulting magnetic domains in the micromagnetic core, thereby producing a magnetically anisotropic device with “easy” and “hard” directions of magnetization, and with corresponding reduction in magnetic core losses at high switching frequencies compared to an isotropic magnetic device. Feygenson further describes depositing a thin chromium and silver film to form a seed layer for further deposition of magnetic material to form a planar magnetic core by an electroplating process that has good adhesion to an insulating oxide layer that is formed on a semiconductor (or other suitable) substrate. The chromium and silver seed layer is etched with a cerric ammonium nitrate reagent without substantial effect on the magnetic alloy.
Filas, et al., in U.S. Pat. No. 6,624,498, entitled “Micromagnetic Device Having Alloy of Cobalt, Phosphorus and Iron,” issued Sep. 23, 2003, which is incorporated herein by reference, describe a planar micromagnetic device formed with a photoresist that is etched but retained between magnetic core and conductive copper layers. The micromagnetic device includes a planar magnetic core of an amorphous cobalt-phosphorous-iron alloy, wherein the fractions of cobalt and phosphorus are in the ranges of 5-15% and 13-20%, respectively, and iron being the remaining fraction. Magnetic saturation flux densities in the range of 10-20 Kilogauss (“kG”) are achievable, and low loss in the magnetic core structure is obtained by depositing multiple insulated magnetic layers, each with a thickness less than the skin depth at the switching frequency of the power converter [e.g., about 2.5 micrometers (“μm”) at 8 megahertz (“MHz”) for relative permeability of μr=1000]. Thin seed layers of titanium and gold are deposited before performing an electroplating process for the magnetic core, and are oxidized and etched without substantial degradation of exposed adjacent conductive copper layers. The planar magnetic core is formed using an electroplating process in an electrolyte with pH about three containing ascorbic acid, sodium biphosphate, ammonium sulfate, cobalt sulfate, and ferrous sulfate. As described by Kossives, et al., in U.S. Pat. No. 6,649,422, entitled “Integrated Circuit Having a Micromagnetic Device and a Method of Manufacture Therefore,” issued Nov. 18, 2003, which is incorporated herein by reference, an integrated device formed on a semiconductor substrate includes a planar magnetic device, a transistor, and a capacitor so that the principal circuit elements of a power converter can be integrated onto a single semiconductor chip.
Thus, although substantial progress has been made in development of techniques for production of a highly integrated power converter that is formed on a single chip, these processes are not suitable for manufacturing an integrated micromagnetic device in substantial numbers and with the process yields and repeatability necessary to produce the reliability and cost for an end product. In particular, electrolytes for forming magnetic and conductive layers should have sufficient life for continued operation in an ongoing manufacturing environment. The electroplating processes should repeatably deposit uniformly thick layers of high-performance magnetic materials with consistent and predictable properties. In addition, the high-frequency ac properties of a micromagnetic core so deposited should exhibit low and repeatable core loss. Similarly, the conductive windings should exhibit low and repeatable high-frequency resistance.
Accordingly, what is needed in the art is a micromagnetic device and method of producing the same that can be manufactured in high volume and with low cost in a continuing production environment, the necessary electroplating tools and electrolytes therefor, and an electroplateable magnetic alloy with high performance magnetic characteristics at switching frequencies that may exceed one megahertz, that overcome the deficiencies in the prior art. In addition, the resulting micromagnetic device should be dimensionally stable with low internal stresses so that the micromagnetic device remains sufficiently planar to support further processing steps.